High temperature furnace



A ril 7, 1964 J- C. ANDERSEN ETAL HIGH TEMPERATURE FURNACE Filed June27. 1960 29 53 4s 47 IO 3 Sheets-Sheet 2 IN VENTORS JAMES C. ANDERSE-NTl MOTHY J. KEATY ATTORNE Y J c. ANDERSEN ETAL 3,128,325

HIGH TEMPERATURE FURNACE April 7, 1 964 5 Sheets-Sheet 3 Filed June 27,1960 INVENTORS JAMES C. AN DERSE N BYT'MOTHY J. KEATY ATTORNEY UnitedStates Patent 3,128,325 HIGH TEMPERATURE FURNACE James C. Andersen,Niagara Fails, and Timothy J. Keaty, Ransomville, N.Y., assignors, bymesne assignments, to the United States of America as represented by theUnited States Atomic Energy Commission Filed June 27, 1960, Ser. No.38,797 Claims. (CI. 13-20) heating elements have had an uppertemperature limit of about 1550" Cl Service above this temperature, evenfor short periods of time, has involved an unacceptable sacrilice inheating element life.

The increasing importance of high temperature refractory materials hasresulted in a growing demand for furnacescapable of operatingsubstantially above 1650 C.

Accordingly it is an important object of the present invention toprovide an electrically heated furnace capable of operating attemperatures up to 1850 C.

Another object is to provide a furnace utilizing silicon carbideelectrical resistance heating elements capable of operation attemperatures up to 1850" C.

Still another object is to provide an electrical resistance heatedfurnace utilizing siliconcarbide heating elements which is capable ofoperation at temperatures up to 1850 C. and having a furnace chamberwherein a variety of inert or oxidizing atmospheres can be provided, asdesired.

Other objects will become apparent to those skilled in the art byreference to the accompanying drawings and specification.

FIGURE 1 is an isometric view of a preferred embodiment of the furnaceof the present invention, with portions being broken away in section;

FIGURE 2 is an enlarged longitudinal sectional view taken along the line22 of FIG. 1;

FIGURE 3 is a reduced cross sectional view taken along the line 33 ofFIG. 2;

FIGURE 4 is an enlarged detail sectional view showing the manner inwhich the furnace chamber is connected to the end wall of the casing ofFIGS, 1, 2, and 3; and

FIGURE 5 is an isometric view of a second embodiment of the furnace ofthe present invention, with portions being broken away in section.

Briefly the furnace of the present invention comprises a hollow graphitemufiler of cylindrical configuration with closed ends, coaxially ofwhich and extending therethrough is positioned ahightemperature-resistant (i.e. refractory) furnace chamber forming tube.Radially spaced from the furnace chamber and parallel thereto areelectrical and thermal conducting silicon carbide heating elements. Coldends of the heating elements extend from each end of the muffle, beingshielded in high temperature-resistant (i.e. refractory) tubes, whichprovide electrical insulation and an enclosure for a protective gasatmosphere.

A gas inlet tube is directly connected to a wall of the muffle foradmission of an inert gas to protect thesilicon carbide heating elementsagainst oxidation and consequent deterioration. The gas bleeds offaround the shrouded cold ends, providingprotection for them also.

Preferably a gas-tight casing surrounds the furnace as sembly and isfilled with an appropriate insulating material.

By reference to the drawings, it will be seen specifically that thepreferred form of the furnace comprises a hollow cylindrical muffle 10,made of graphite, which is an eleczontal angles 14. The vertical anglesextend downwardly to form support feet 15. The frame elements 12 arepositioned inwardly of the ends of the casing 11. The casing frame alsoincludes horizontally disposed angles 16 which extend beyond the frameelements 12 and are joined with end vertical angles 17. Horizontalangles 18 connect the ends of the upper and lower horizontal angles 16to complete the framework. 2

The center portion of the top of the casing consists of a sheet metalplate 19 secured in position to the frame by bolts 19 and a gasket 19".This provides access to the interior for assembly of the furnace. Topend plates 19" are fastened by gas-tight welds. Bottom plates 20 arealso welded to the frame, as are side plates 21. Partition walls 22 arefastened to the frame elements 12 in gas-tight relationship, as bywelding. These are positioned inwardly of the ends of the casing toprovide access chambers 23. End plates 24 are'attached to the endverticle angles 17 and horizontal angles 18. These are removably securedas by bolts 25 so that they can be removed to open the access chambers23. The end plates 24-are fitted in gastight relationship by a gasket26.

The muffie 10 is placed within the casing 11 and is supported centrallythereof by a U-shaped support comprising a base block 27, resting on thebottom of the casing, andtwo upright blocks 28. The hollow mufiie isprovided with apertured ends 29. As best shown in FIG. 3 the ends areeach'provided with a central aperture 30 posi tioned coaxially of themufile. Also the ends are provided with a plurality of apertures 31spaced equally circumferentially and radially from the axis of themuffle and concentric to the central aperture 30. It is to be noted inFIG. 2 that the aperture patterns at each end of the mufiie areidentical and the apertures in each end are in aligned relationship.

As best shown in FIG. 2, an isolated furnace chamber forming tube 32 inthe preferred form of a high density refractory tube of alumina isfitted into the central apertures 30 and extends through the mufile,with the ends thereof extending through aligned holes 33 provided in thepartition walls 22 and also aligned holes 34 provided in the end plates24. The clearance between the holes in the muffle and the partitionwalls and the alumina tube is made as a reasonably tight mechanical fit.However, the alumina tube is sealed in the holes 34 in the end plates,as shown in the enlarged sectional view of FIG. 4 to provide a gas-tightseal.

The combined cooling and sealing means comprises an annular washer 35which is welded gas tightly as at 36 to the end plates 24 in alignedrelationship to the aperture 34. A double-walled tubular sleeve 37 isfitted into the aperture of the washer and is gas tightly secured bywelding 38. The tubular sleeve includes a cavity 39 through which acoolant such as water is circulated. Inlet and outlet tubes 40 and 41are connected to the cavity 39 in coolant sealing relationship and areconnected to a suitable source of coolant (not shown) to provide thenecessary cooling. The tubular sleeve 37 is provided with an inwardlytapered mouth 42. The alumina tube 32 is fitted through the interior ofthe sleeve 37 and extends a short distance beyond the end thereof. Aheat-resistant annular gasket 43 of pliable material is fitted betweenthe alumina tube and the mouth 42 of the sleeve. A cap 44 is fitted overthe end of the sleeve 37 as by threading and compresses the gasket 43 ingas-tight relationship between the alumina tube and the tapered mouth ofthe sleeve. High temperature-resistant (i.e. refractory) sbieldmg tubes,such as alumina tubes 45 are also fitted tightly in the remainingapertures 31 in the end walls 29 of the muflle and extend through andjust beyond the partition walls 22 in appropriate apertures 46 providedtherein. These tubes do not extend through the muffie as does thefurnace chamber; instead, they are fitted into the ends of the mufflewith one end approximately flush with the inside of the end wall of themufiie. The apertures in the partition walls are formed in the samepattern as that of the end walls of the mufile and all aperture patternsare aligned.

Silicon carbide heating elements 47 are removably positioned in thealumina tubes 45 and slidably extend through these tubes and the muffleIt These heating elements include a central heating section 48, withsocalled cold ends 49 joined thereto. The heating sections are exposedwithin the interior of the muifie and are in surrounding, radiant-heattransfer relationship to the furnace chamber formed by tube 32. The coldends lie outside of the mufiie but are partially encased within thealumina tubes :5. Electrical connections are made to exposed portions ofthe cold ends of the heating ele ments, which protrude into the accesschambers for such purpose. Power is brought to terminals 50 from asuitable source thence to straps 5i? to heat the resistance elements 47.

As shown in FIGURES 1 and 3, a sight tube 51 extends through the casing11 and is directly connected at its inner end to the mufiie Iii. Thesight tube is shown as being provided with a sight glass 52 and a cap 53threadably retaining the glass in position. A gas inlet tube 54- isjoined to the sight tube and a selected inert gas is ad mittedtherethrough to fill the interior of the inuflie. This gas surrounds theheating elements, protecting them from oxidation, and bleeds oif throughthe alumina tubes 45 surrounding the cold ends of the elements and alsothereby protecting them from deterioration by oxidation.

The interior space 55 of the casing 11, surrounding the mufile it? andassociated parts, between the partition walls 22, is filled with aparticulate mass of suitable thermal insulating material such as carbonblack. An outlet tube 56 permits the inert gas to exhaust from thecasing.

A second embodiment of the furnace of the present invention isillustrated in FIGURE 5. This embodiment is essentially the same as thepreferred embodiment of FIGURES 14, except that the casing 57 isfabricated of firebrick 58. In this embodiment the interior space of thecasing surrounding the mufiie and associated parts is filled with aparticulate mass of thermal insulating material comprising fine siliconcarbide and carbon. The carbon addition retards oxidation of the siliconcarbide and inhibits formation of low melting silica glass.

In this embodiment the alumina tubes 45 surrounding the heating elements4'7 are notched into the bricks 58 of the end walls of the casing 57 toprovide support and the cold ends .9 of the silicon carbide resistanceheating elements extend through suitable holes provided in the bricks,which provide electrical insulation. Appropriate electrical connectionsare made to the exposed ends of the heating elements in the manner ofthe preferred embodiments described above. A blanket 60 of hightemperature-resistant fibrous material, such as aluminum silicate fibersis placed over the insulation material when the casing is open toprovide access to the interior for assembling the furnace therein.

The invention is highlighted by the following examples:

EXAMPLE I A furnace was constructed in accordance with the presentinvention having a dense alumina furnace tube 1 /8 inside diameter. Theheating elements were 28" x 8" x The heating elements were regularlyspaced around the exterior of ther" urnace tube about 3% inches oncenter therefrom. Heat transfer of the silicon carbide heating elementsresulted in only a 55 C. differential between the surface temperature ofthe element and the interior of the furnace tube at 1850 C.

EXAMPLE II A furnace was constructed in accordance with the conceptillustrated in FIGURES 1, 2, 3 and 4 of the drawings using carbon as aninsulation around the muffie. A run employing low watt-loading and highnitrogen flow, resulted in 518 hours of continuous service at 1800 C.The furnace was returned to service merely by replacing the necessaryheating elements.

The high thermal eificiency of the carbon insulation minimizeselectrical power requirements. For example, an ambient temperature of1750 C. can be maintained at a power input of 2.2 kw. This means thatexpensive high current transformers are not required. Power requirementsfor various operating temperatures are shown in Table I below.

1 1:Qurface loading in watts per square inch of nominal elementradiating sur ace.

EXAMPLE III.-FURNACE PERFORMANCE Two furnaces of the type shown inFIGURE 5 of the drawings have been extensively used in laboratory work.One furnace was in continuous service for 150 hours at 1750 C. beforeheating element failure. The other furnace was operated intermittentlyfor a total of hours between 1600 and 1850 C. before one of the elementsfailed. The intermittent service constituted 40 cycles from roomtemperature to operating temperature with heat-up times of two to fourhours. Both furnaces were restored to service by simply replacing thenecessary heating elements.

Gases which can be used to form the protective atmosphere in furnaces ofthe present invention include nitrogen, argon and helium. Nitrogen ispreferred from an economic standpoint; however, in the temperature range1350-1650 C., argon or helium is preferred, because nitrogen reacts withsilicon carbide to form silicon nitride in this temperature range.

In operating furnaces of the present invention it is preferred to keepthe watt-loading per unit area of nominal element radiating surface at aminimum, preferably less than 20-25 watts per square inch. This is toprevent overheating of the current carrying silicon carbide bridgeswhich leads to premature element failure. This low wattloading is easilyaccomplished in accordance with the present invention because of thehigh thermal efiiciency of the carbon insulation, which was referred toabove as surrounding the muffie within the embodiment of FIGURES l-4.Also the silicon carbide-carbon mixture utilized in FIGURE 5 is alsohighly efficient and provides for the desired low watt-loading.

Another insulating material, in addition to carbon and carbon-siliconcarbide mixtures already referred to, is alumina. Alumina in the form ofsmall, hollow spheres is particularly desirable.

While the preferred material from which the furnace chamber isfabricated is high density, non-porous alumina, other materials such aszirconia can be employed.

In addition to the alumina tubes which are utilized to encase the coldends of the heating elements, silicon carbide tubes are contemplated forthis purpose for reasons of economy. However, since silicon carbide iselectrically conductive, short alumina sleeve or washer-like electricalinsulators will be used at each end of the tube to provide the desiredelectrical insulating interior surfaces and thereby keep the heatingelement out of contact with the tube. 7

Although it has been mentioned above, a particular advantage of thefurnace of the present invention is that when a heating element fails,the furnace does not have to be rebuilt to return it to operatingcondition. By virme of the tubes in which the fcold ends of the heatingelements are supported, heating elements can be replaced merely bysliding them out of their position and placing a new element therein.Thus a furnace is provided which i is characterized by long andeflicient operation at severe temperatures greatly exceeding thoseheretofore known in the art.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention, following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth, and as fall within the scope of theinvention or the limits of the appended claims.

We claim:

1. An electrical resistance furnace comprising a hollow casing formed ofstructural material such as metal and having gas outlet means and aninterior closed by end walls; a hollow muflie formed of thermalinsulating refractory material such as graphite and arranged within saidinterior and having an interior closed by end walls, said casing andmufiie end walls being provided with alined central apertures and alinedouter apertures surrounding said central apertures, an elongated tubeformed of dense refractory material such as alumina and extendingthrough said mufile and easing, said tube fitting tightly in saidcentral apertures in said casing and muffle end walls and forming anisolated furnace chamber, elongated shielding tubes formed of refractorymaterial such as alumina and extending between said casing and mufileend walls, said shielding tubes fitting tightly in said outer aperturesin said casing and muflle end walls and provided with interior surfacesof electrical insulating material such as alumina, a particular massformed of thermal insulating refractory material such as carbon andfilling said casing interior around said muffle, furnace tube andshielding tubes, removable elongated heating elements formed ofelectrical conducting refractory material such as silicon carbide andslidably extending through said mufile and shielding tubes, said heatingelements including central heating sections exposed within said rnufileinterior in radiant heat transfer relationship with said furnace tubeand cold end portions arranged within said shielding tubes and therebyelectrically insulated from said mufile and mass, and a gas inlet tubedirectly connected to said muflle'interior for providing an inertatmosphere surrounding said central heating sections and for permittingsaid atmosphere to bleed off around said cold end portions through saidshielding tubes in order to effectively protect said heating elementsagainst oxidation, whereby said furnace is capable of operating attemperatures up to about 1850" C., with only about a 55 C. differentialbetween the surface temperature of said heating elements and theinterior of said furnace tube, with a surface loading of less than about25 watts per square inch of nominal heating element radiating surface,and with a power input requirement of less than about 3 kilowatts.

2. An electrical resistance furnace as in claim 1 wherein said casingmaterial is selected from the group consisting of metal and firebrick,said muflle material is graphite, said furnacetube material is alumina,said shielding tube material is selected from the group consisting ofalumina and silicon carbide, said shielding tubes of silicon carbideincorporating internal insulators of alumina to form said internalsurfaces and thereby electrically insulate said heating elements fromsaid silicon carbide shielding tubes as well, said mass material isselected from the group consisting of carbon, a mixture of carbon andsilicon carbide, and alumina, and said heating element material issilicon carbide. I g i 3. An electrical resistance furnace comprising ahollow substantially gas-tight casing formed of metal and having gasoutlet means, a central interior closed by inner end walls and outeraccess chambers closed by outer end walls, a hollow muflie formed ofgraphite arranged within said central interior and having an interiorclosed by end walls, said inner and outer'end walls and said muffle endwalls being provided with alined central apertures and said inner endwalls and muffle end walls being provided with alined outer aperturessurrounding said central apertures therein, combined sealing and coolingmeans fitting substantially gas tightly in said central apertures insaid outer end walls, an elongated tube formed of dense alumina andextending through said mufiles and casing, saidtube fitting tightly insaid central apertures in said inner end walls and muffle end walls andsubstantially gas tightly in said combined sealing and cooling means andforming an isolated furnace chamber, shielding tubes formed of aluminaand extending between said inner end walls and muffle end walls andfitting tightly in said outer apertures therein, a particulate massformed of carbon black and filling said central interior around saidmuffle, furnace tube and shielding tubes, removable elongated heatingelements formed of silicon carbide and slidably extending through saidmuffle and shielding tubes into said access chambers, said elementsincluding central heating sections exposed within said muffle interiorin radiant heat transfer relation to said furnace tube and cold endportions arranged within said shielding tubes and thereby electricallyinsulated from said mufile, mass and inner end walls, and a gas inlettube directly connected to said muflie interior for providing an inertatmosphere surrounding said central heating sections and for permittingsaid atmosphere to bleed off around said cold end portions through saidshielding tubes in order to effectively protect said heating elementsagainst oxidation, whereby said furnace is capable of operation attemperatures up to about 1850 C., with only about a 55 C. differentialbetween the surface temperature of said heating elements and theinterior of said furnace tube, with a surface loading of less than about25 watts per square inch of nominal heating element radiating surface,and with a power input requirement of less than about 3 kilowatts.

4. An electrical resistance furnace as in claim 3 wherein each of saidcombined cooling and sealing means; includes a double walled tubularsleeve fitting substantially gas tightly in said central aperture in thecorresponding one of said outer end walls, said sleeve having a cavitythrough which a cooling medium is circulated, a threaded outer surfaceat one end and an inwardly tapered mouth at said one end, a heatresistant gasket arranged between said mouth and said furnace tube, andan internally threaded cap compressing said gasket into substantiallygas tight relationship between said mouth and furnace tube.

5. An electrical resistance furnace comprising a hollow casing formed offirebrick and having an interior closed by end walls, a hollow muffleformed of graphite and arranged within said interior and having aninterior closed by end walls, said casing and muflle end walls beingprovided with alined central apertures and alined outer aperturessurrounding said central apertures, an elongated tube formed of densealumina and extending through said muffle and easing, said tube fittingtightly through said central apertures in said casing and mufile endwalls and forming an isolated furnace chamber, elongated shielding tubesformed of alumina and extending between said casing and muflie end Wallsand fitting tightly in said outer apertures therein, a particulate massformed of a mixture of carbon and silicon carbide and filling saidcasing interior around said muffle, furnace tube and shielding tubes,removable elongated heating elements formed of silicon carbide andslidably extending through said mufiie and shielding tubes, said heatingelements including central heating sections exposed Within said mufileinterior in nadiant heat transfer relation to said furnace tube and coldend portions arranged in said shielding tubes and thereby electricallyinsulated from said muffle and mass, and a gas inlet tube directlyconneeted to said mufiie interior for providing an inert atmospheresurrounding said central heating sections and for permitting saidatmosphere to bleed ofif around said cold end portions through saidshielding tubes in order to efiectively protect said heating elementsagainst oxidation, whereby said furnace is capable of operation attemperatures up to about 1850 C. with only about a 55 C. differentialbetween the surface temperature of said heating elements and theinterior of said furnace tube, with a surface loading Olf less thanabout 25 watts per square inch of nominal heating element radiatingsurface, and with a power input requirement of less than about 3kilowatts.

References Cited in the file of this patent UNITED STATES PATENTS1,528,542 Hancock et a1. Mar. 3, 1925 1,832,872 Millar Nov. 24, 19311,837,179 Benner et a1. -2 Dec. 15, 1931 1,903,036 Francis Mar. 28, 19332,294,034 Jaeger Aug. 25, 1942 2,423,021 Henckler et a1. June 24, 19472,881,297 Friedman Apr. 7, 1959 FOREIGN PATENTS 227,223 Great BritainIan. 15, 1925

1. AN ELECTRICAL RESISTANCE FURNACE COMPRISING A HOLLOW CASING FORMED OFSTRUCTURAL MATERIAL SUCH AS METAL AND HAVING GAS OUTLET MEANS AND ANINTERIOR CLOSED BY END WALLS; A HOLLOW MUFFLE FORMED OF THERMALINSULATING REFRACTORY MATERIAL SUCH AS GRAPHITE AND ARRANGED WITHIN SAIDINTERIOR AND HAVING AN INTERIOR CLOSED BY END WALLS, SAID CASING ANDMUFFLE END WALLS BEING PROVIDED WITH ALINED CENTRAL APERTURES AND ALINEDOUTER APERTURES SURROUNDING SAID CENTRAL APERTURES, AN ELONGATED TUBEFORMED OF DENSE REFRACTORY MATERIAL SUCH AS ALUMINA AND EXTENDINGTHROUGH SAID MUFFLE AND CASING, SAID TUBE FITTING TIGHTLY IN SAIDCENTRAL APERTURES IN SAID CASING AND MUFFLE END WALLS AND FORMING ANISOLATED FURNACE CHAMBER, ELONGATED SHIELDING TUBES FORMED OF REFRACTORYMATERIAL SUCH AS ALUMINA AND EXTENDING BETWEEN SAID CASING AND MUFFLEEND WALLS, SAID SHIELDING TUBES FITTING TIGHTLY IN SAID OUTER APERTURESIN SAID CASING AND MUFFLE END WALLS AND PROVIDED WITH INTERIOR SURFACESOF ELECTRICAL INSULATING MATERIAL SUCH AS ALUMINA, A PARTICULAR MASSFORMED OF THERMAL INSULATING REFRACTORY MATERIAL SUCH AS CARBON ANDFILLING SAID CASING INTERIOR AROUND SAID MUFFLE, FURNACE TUBE ANDSHIELDING TUBES, REMOVABLE ELONGATED HEATING ELEMENTS FORMED OFELECTRICAL CONDUCTING REFRACTORY MATERIAL SUCH AS SILICON CARBIDE ANDSLIDABLY EXTENDING THROUGH SAID MUFFLE AND SHIELDING TUBES, SAID HEATINGELEMENTS INCLUDING CENTRAL HEATING SECTIONS EXPOSED WITHIN SAID MUFFLEINTERIOR IN RADIANT HEAT TRANSFER RELATIONSHIP WITH SAID FURNACE TUBEAND COLD END PORTIONS ARRANGED WITHIN SAID SHIELDING TUBES AND THEREBYELECTRICALLY INSULATED FROM SAID MUFFLE AND MASS, AND A GAS INLET TUBEDIRECTLY CONNECTED TO SAID MUFFLE INTERIOR FOR PROVIDING AN INERTATMOSPHERE SURROUNDING SAID CENTRAL HEATING SECTIONS AND FOR PERMITTINGSAID ATMOSPHERE TO BLEED OFF AROUND SAID COLD END PORTIONS